Rock bolt assembly and procedure for use in conjunction with blasting operations



Aprll 23, 1968 c. i. WILLIAMS 3,379,016

ROCK BOLT ASSEMBLY AND PROCEDURE FOR USE IN CONJUNCTION WITH BLASTINGOPERATIONS Filed Jan.

INVENTOR. CHESTER W/L L/A M5 United States Patent ROCK BOLT ASSEMBLY ANDPROCEDURE FOR USE IN CONJUNCTION WITH BLAST- ING OPERATIONS Chester I.Williams, 347 Greeubriar, SE., Grand Rapids, Mich. 49506 Filed Jan. 11,1965, Ser. No. 424,657 5 Claims. (Cl. 61-35) ABSTRACT OF THE DISCLOSUREA rock bolt with removable shielding around it along the portionimmediately inward from the surface of a rock formation in which thebolt is installed, the shield being operative to prevent the bonding ofgrout to the shielded portion of the bolt.

This invention relates to rock bolt installations adapted for use in anarea where blasting operations are performed. It is common practice toinstall rock bolts in a ground formation prior to blasting in order toprevent or control the displacement of certain portions of the rock. Todevelop the full strength of these rock bolts, it is also the usualpractice to charge the hole in which the rock bolt is installed with acementitious composition commonly known as grout to increase the bondbetween the rod and the surrounding ground formation, and also toprotect the bolt against corrosion. To obtain the maximum benefits fromthe bolt, a pre-stressing operation should be performed which has theeffect of stretching the rod sufiiciently to generate stressesapproaching the normal working load, so that these forces can be appliedto the ground formations to hold them in position. Withoutpre-stressing, the ground formation must exhibit considerabledisplacement before the rock bolt is actually able to apply itsrestraining forces.

When blasting operations are performed, there is usually enoughdisturbance of the ground formation adjacent the outer end of the rockbolt so that the nut is no longer tight against the bearing plate, thusreducing the effectiveness of the bolt. Re-tightening of the bolt isvery difficult to accomplish, if at all possible, without destroying thebond between the bolt and the previously-injected grout. It alsofrequently happens that the detonation of a blasting charge loosens asuflicient quantity of the ground formation so that some of the areaimmediately adjacent the bearing plate is removed entirely. In the usualrock bolt having a few inches of threading at the outer end, about theonly way in which the rock bolt could be made to continue to perform aportion of its function would be to weld the bearing plate to the rod,and then cut off the projecting useless extremity of the rod. This, ofcourse, would not leave the outer portion of the bolt rod in a stressedcondition.

Applicant has devised a rock bolt assembly which can be used inconjunction with blasting operations to permit a secondary prestressingwithout the necessity of pulling the outer extremity of the bolt rodthrough a surrounding grout bond as the secondary stressing operation isperformed. This assembly also has the desirable characteristic ofrendering the outer section of the bolt rod replaceable so assubstantial removal of the ground formation can be accommodated byinstalling a shorter section of bolt rod in place of the original outerportion.

The several features of the invention will be analyzed in further detailthrough a discussion of the particular embodiment illustrated in theaccompanying drawing. In the drawing:

FIGURE 1 is a sectional elevation showing the installed condition of arock bolt assembly embodying this ice invention, in a condition such aswould be encountered prior to a blasting operation.

FIGURE 2 illustrates the condition of the rock bolt assembly after theblasting operation has been performed, and a new outer section of thebolt rod has been installed.

FIGURE 3 is a section taken on the plane 3-3 of FIGURE 1.

Referring to the drawing, the illustrated rock bolt assembly is showninstalled in a hole 10 in the rock formation 11. The bolt rod has anouter portion 12 and an inner portion 13 joined in threaded engagementby the coupling 14. The inner end of the portion 13 is provided with ananchor device of any convenient form. In the illustrated modification, aconical expander 15 has threaded engagement with the inner end of theportion 13, and is surrounded by the expansible shell 16. Axial movementof the expander 15 to the left in response to the rotation of the boltrod will induce expansion of the shell 16, with the thrust resultingfrom this operation being taken by the ring 17 which remains axiallyfixed with respect to the bolt rod.

It is usual practice to surround the bolt rod with a charge of groutgenerally indicated at 18. To eliminate a bond between the outerextremity of the bolt rod, the illustrated assembly includes a shieldingtube 19 preferably extending over the full length of the outer portion12. This shielding tube may be of cardboard or any other convenientfrangible material, the primary function of which is to retain the groutoutside of the tube so that it does not come into contact with the boltrod. A coating may be applied to the rod itself to perform thisfunction, with the characteristic of this coating providing that the rodcan move axially to some extent without disturbing the grout. This canbe done either by permitting the rod to slip within the protectivecoating or tube, or by providing that the shield material itself hassufficient plasticity to deform in shear and permit the necessaryrelative axial movement as the bolt rod is stressed.

The outer portion of the bolt assembly is conventional, and includes thebearing plate 20, the bevel washer 21, and the nut 22. Pro-stressing ofthe rock bolt assembly is accomplished by a rotation of the nut 22 withsufficient torque to develop a substantial portion of the tensilestrength of the bolt rod. The assembly is usually grouted in thispro-stressed condition.

After the blasting operation has taken place, the usual result is asufficient disturbance of the outer portion of the rock formation torender the nut 22 fairly loose. The prevention of a bond between thegrout and the outer portion of the rod will permit the elasticshortening of the rod that is inevitable upon such release of stresses,without disruption of the surrounding grout. This elimination of bondwill also permit the nut 22 to be re-tightened so that the rock boltassembly can be pre-stressed again in the final condition of the groundformation.

In situations where the blasting operations have resulted in theloosening and removal of the rock at the surface, it is possible todisconnect the outer portion 12 of the bolt rod and replace it byanother shorter portion 120, followed by the re-tightening of the nut 22as described above. Under normal conditions, the outer portion 12 can beunscrewed, and the projecting part of the tube 19 cut off generallyflush with the new surface of the ground formation. A mass of plasticsealing material indicated at 23 will of course not be present in thenew condition, since that portion of the rock formation will have beenremoved. It is most convenient to extend the shielding tube 9 down tothe coupling 14, which should represent approximately the outer four tosix feet of the bolt length, at least. The coupling therefore forms aconvenient stop establishing the axial position of the shielding tube.The tube also forms a protective guide for the insertion of a new outerportion 12a of the bolt rod, since it will form a conduit leadingdirectly to the coupling 14 which will not be contaminated with rockchips, particles of grout, or other foreign material.

The particular embodiment of the present invention which has beenillustrated and discussed herein are for illustrative purposes only andare not to be considered as a limitation upon the scope of the appendedclaims. In these claims, it is my intent to claim the entire inventiondisclosed herein, except as I am limited by the prior art.

I claim:

1. A rock bolt assembly, in combination with a mass of grout surroundingsaid assembly in a hole in a ground formation, said assembly comprising:

a bolt rod;

an anchor device secured to the normally inner end of said bolt rod andsaid ground formation; and

a shield material of substantially less strength than the material ofsaid bolt rod surrounding exclusively only an outer portion of the saidbolt rod whereby said portion is isolated from said grout, said groutbeing in contact with substantially all of the remaining inner portionof said rock bolt assembly.

2. An assembly as defined in claim 1, wherein said rod has an outer andan inner portion connected by a coupling.

3. An assembly as defined in claim 2, wherein said shield material is atube extending over said outer portion to said coupling.

4. A method of conducting blasting operations in a ground formation,comprising:

drilling at least one hole in said formation, and installing a rock bolttherein and grouting the same in place, with the outer portion thereofcovered by a shield material;

detonating a charge of explosive in the adjacent ground formation; and

tightening said rock bolt after said charge has exploded.

5. A method as defined in claim 4, wherein said rock bolt has inner andouter portions connected by a coupling, and tightening said rock boltincludes replacement of said outer portion with a shorter outer portion.

References Cited UNITED STATES PATENTS 1,883,196 10/1932 Wertz 61-45 X2,442,113 5/1948 Beij 6145 X 3,226,933 1/ 1966 White 6139 FOREIGNPATENTS 152,879 1953 Australia.

80,182 1963 France. 341,185 11/1959 Switzerland.

OTHER REFERENCES DAS, 1,104,471, 4-1961, German printed application toEckert.

JACOB SHAPIRO, Primary Examiner.

